1. Field of the Invention
The present invention relates to a semiconductor laser chip, and to a method of formation thereof. More specifically, the invention relates to a semiconductor laser chip provided with a ridge portion, and to a method of formation thereof.
2. Description of Related Art
Compound semiconductor laser chips are generally provided with a stripe-shaped (long, narrow) ridge portion as a waveguide structure. The ridge portion is formed by etching a semiconductor element layer composed of a plurality of semiconductor layers laminated on a substrate.
In this case, the etching depth affects the light confinement in the horizontal-lateral direction in the light waveguide, and therefore, in etching for forming the ridge portion, accuracy of the controllability of etching is strongly demanded. It should be noted that, if the controllability of etching for forming the ridge portion is low, and thus the distance from the etching bottom surface to the active layer at the side of the ridge portion varies, the light confinement effect in the horizontal-lateral direction at the side of the ridge portion varies. And if the light confinement effect in the horizontal-lateral direction at the side of the ridge portion varies, disadvantageously, the controllability of the light radiation angle in the horizontal direction in the semiconductor laser chip lowers.
For this reason, there have conventionally been proposed methods for improving the controllability of etching depth in etching for forming the ridge portion (see, for example, Non-patent Document 1).
Non-patent Document 1 mentioned above discloses a method according to which, in forming a semiconductor element layer, an etching marker layer is formed beforehand at the position (end position) at which to stop etching, and laser light is shone on the surface of the semiconductor element layer so that, while the reflected light is being monitored, the semiconductor element layer is etched by dry etching until, at the time point that the etching marker layer is detected, etching is stopped.
It should be noted that, in the method disclosed in Non-patent Document 1 mentioned above, when the wavelength of the laser light shone on the semiconductor element layer is set shorter than the absorption edge of the etching marker layer, the interference behavior of the reflected light is strongly affected by the light absorption at the etching marker layer. More specifically, when laser light is shone on the semiconductor element layer having a plurality of semiconductor layers laminated, due to the differences in index of refraction among the semiconductor layers, interference occurs in the reflected light; when the etching depth reaches the etching marker layer, the amount of light absorbed by the etching marker layer changes, and accordingly the interference behavior of the reflected light changes. Thus, by detecting a change in the interference behavior of the reflected light, it is possible to check whether or not the etching depth has reached the end position (etching marker layer).
Non-patent Document 1: Photonics Technology Letters, Vol. 2, p. 697
With the method disclosed in Non-patent Document 1 mentioned above, since the end position of etching is detected based on a change in the interference behavior of the reflected light, when a change in interference behavior is detected, the etching depth has reached the etching marker layer. Moreover, to suppress absorption by the etching marker layer of the light produced at the active layer, the thickness of the etching marker layer is generally set as small as 50 nm or less. Thus, after the end position of etching is detected, the etching marker layer is etched in an extremely short time. Thus, even when etching is stopped on detection of a change in the interference behavior of the reflected light, as shown in FIG. 25, the semiconductor element layer is etched beyond the etching marker layer 400 into the semiconductor layer 401 under it.
On the other hand, in a case where a semiconductor element layer is etched by dry etching, after formation of the ridge portion, chemical etching (wet etching) is generally performed to remove reaction products produced during etching and parts damaged by etching. In a case where the ridge portion is formed by the method disclosed in Non-patent Document 1 mentioned above, as described above, the semiconductor element layer is etched beyond the etching marker layer 400 into the semiconductor layer 401 under it; thus, when chemical etching is performed after formation of the ridge portion, due to the difference between the etching speed at the etching marker layer and that at the semiconductor layer around it, after the completion of etching, inconveniently, as shown in FIG. 26, the etching marker layer 400 protrudes from the side surfaces of the ridge portion 402 or, as shown in FIG. 27, the etching marker layer 400 recedes from the side surfaces of the ridge portion 402. Inconveniently, due to such protrusion or recession of the etching marker layer, surface irregularities are produced in the side surfaces of the ridge portion. As a result, when, in a step after formation of the ridge portion, an insulating layer or an electrode layer is formed on the side surfaces of the ridge portion, disadvantageously, those surface irregularities in the side surfaces of the ridge portion produce layer discontinuity in the insulating layer or electrode layer formed.